1. Field of Invention
This invention relates to a highly active catalyst useful for syngas generation, and more particularly to a calcium promoted, nickel catalyst on an alumina support, wherein the catalyst is stabilized by the addition of titanium. The catalyst is highly active and resistant to coking especially in a feed stream containing significant quantities of CO and CO2, relatively low quantities of steam, and, optionally, relatively high quantities of sulfur compounds. A process of manufacture of the catalyst and a process of use of the catalyst are also disclosed.
2. Background Art
Production of synthesis gas or syngas (various blends of gases generally comprising hydrogen and carbon monoxide) is an important process step in the manufacture of numerous chemicals, such as ammonia and methanol. It is also useful in numerous other commercial processes, such as iron ore reduction, Fischer-Tropsch synthesis and gas-to-liquid technology. Many of the synthesis gas plants produce the syngas by steam reforming hydrocarbons. Typically, these plants employ a process of catalytic steam reforming of methane in the presence of a supported nickel catalyst, usually nickel on an alumina support or nickel on a promoted alumina support.
However, the presence of sulfur compounds in the reforming system, for example, quantities of H2S as low as several parts per billion, can deactivate conventional steam reforming catalysts. Therefore, sulfur is usually removed before being allowed to enter the reformer. In a reducing gas generation process, the feed may include a significant amount of sulfur or sulfur compounds, and the catalysts need to retain sufficient high reforming activity at sulfur levels up to about 20 parts per million. While higher temperatures and H2 partial pressures in the reaction feed can reduce the level of deactivation, these higher temperatures may also adversely affect the physical structure of the steam reforming catalysts.
Another problem that often occurs with reforming reactions is an enhanced likelihood of coking or carbon formation on the catalysts. In conventional reforming processes, there is essentially no CO in the feed stream. In contrast, in reducing gas generation processes, the low H2O and high CO and CO2 conditions make coking of the reforming catalysts a problem. Carbon formation from CO at the inlet section of the reformer in the reducing gas generation process is especially critical. On nickel catalysts the effect of this coking is coating of the active nickel sites and plugging of the pores of the catalyst.
Several solutions have been proposed to address the coking problem. For example, manufacturers have used a large excess of H2O in the reformer feed stream, but this is not suitable for reducing gas generation processes. U.S. Pat. No. 5,753,143 proposes the use of a noble metal catalyst. It is well known that noble metal catalysts have higher coke formation resistance compared to conventional steam reforming catalysts that merely utilize nickel, but these noble metal catalysts are quite expensive, especially with the large quantity of catalysts that is conventionally utilized for this type of reaction. Morioka has addressed the coking problem by the use of high dispersion of metal species over the surface of the catalyst, such as various types of double hydroxide catalysts. U.S. Pat. No. 4,530,918 teaches a nickel on alumina catalyst with a lanthanum additive.
Another process for limiting coke formation on nickel catalysts during reforming reactions utilizes the sulfur that is naturally present in the feed stream. In this process—referred to as passivation—sulfur poisons some, but not all, of the nickel sites on the catalyst and produces a reforming catalyst which retains sufficient active sites to be useful for gas production at lower H2/CO ratios. The amount of sulfur that is present in the feed stream must be carefully controlled so that the catalyst retains sufficient activity for the reforming reaction, and the process often requires a substantial quantity of catalyst in the bed.
Conventional steam reforming nickel on alumina catalysts may include additives to enhance their performance and to reduce the coking problem. For example, alkali compounds may be added to steam reforming catalysts to reduce carbon formation but because of their potential migration during high temperature processing the alkali metals can adversely impact downstream operations. Magnesia has also been added to steam reforming catalysts to suppress carbon formation, but magnesia promoted catalysts are hard to reduce and maintain in a reduced state.
Nickel oxide and magnesia are very similar in structure. Thus, a nickel oxide and magnesia combination material is usually formed during the high temperature reaction. The reducibility of nickel oxide and the activity of a magnesia-based catalyst is heavily dependent on the calcination temperature and opertion conditions, with a less active catalyst resulting when the calcination temperature is higher than 400° C. and in a less reducing environment. Thus, magnesia-supported nickel catalysts are difficult to utilize for reforming reactions, especially—for reducing gas generation.
A nickel catalyst for reducing gas generation is conventionally produced by impregnating nickel on an alumina or magnesia carrier. In use because the reforming reaction is a strongly endothermic reaction and in order to obtain high hydrocarbon conversion, high temperatures are required for the reaction, sometimes running as high as 1000° C. Even when the reaction is conducted at lower temperatures, in the range of 700° C., it is still necessary to use low surface area alumina, such as alpha alumina as the carrier material for these catalysts. In fact, alpha alumina is the only alumina phase that is stable enough to be used as a carrier under conventional reforming conditions. With catalysts produced from alpha alumina carriers, however, the BET surface area, pore volume and nickel dispersion on these catalysts is quite low. For example, a conventional steam reforming catalyst of this type prepared with nickel on alpha alumina has a BET surface area in the range of 1-4 m2/g, a pore volume from about 0.08 to 0.16 cc/gm and a nickel specific surface area from about 0.5 to 1.5 m2/g.
While lanthanum-promoted alumina catalysts of U.S. Pat. No. 4,530,918 have shown some advantages in the production of carbon monoxide rich syngas at close to stoichiometric requirements, the surface area and nickel dispersion of these catalysts is still in a range comparable to conventional alpha alumina-based steam reforming catalysts with BET surface areas only slightly improved to about 5 m2/gm with a nickel specific surface area less than 2 in2/g.
Thus, there is still a need to improve existing nickel on alumina catalysts for reforming reactions utilizing a feed stream containing significant quantities of CO and CO2 and low quantities of steam (the feed stream having a H2O/CH4<0.8 and a CO2/CH4>0.5). Further, the addition of additives to these catalysts has not to date shown sufficient satisfactory results to overcome the coking problems while maintaining high reforming activity in the presence of a significant amount of sulfur.